The invention relates to a method for electronically imaging the potential distribution in an electronic component by contactless potential measurements with a scanning electron microscope, the primary electron beam of which is keyed and is directed, through deflection, in lines across the component, particularly an integrated circuit, and for display as a potential contrast on a picture screen.
It is known that for functional testing of electronic components with a scanning electron microscope, a method which is called "voltage coding" in the literature can be used. The potential distribution of the component is then imaged on a television screen by having the primary electron beam scan, interlaced, the component in accordance with television principles. The electronic component, for instance, an integrated circuit, is addressed by signals, the frequency of which is a multiple of the line frequency of the picture screen.
At the measuring point, the primary electron beam releases secondary electrons from the metallic conductor, the number of which is determined by the potential of the measuring point. At a measuring point with positive potential, relatively few secondary electrons are released; this results in a correspondingly low brightness on the picture screen. Zero potential or negative potential at the measuring point results in a correspondingly high brightness on the picture screen, and the potential distribution can therefore be made visible by this potential contrast on the picture screen.
Television picture screens work with a line frequency of about 15 kHz. The electron beam is generally led over the picture screen in 625 lines. While scanning an integrated circuit, the potential distribution of which changes, for instance, with a frequency of 60 kHz, the primary electron beam detects the potential change 8 times during a line sweep. The signals displayed on the picture screen can be correlated in time, and any time displacement of the potential can be recognized visually. However, due to the relatively low signal-to-noise ratio, television quality of the picture is not achieved. It therefore becomes necessary to adjust a relatively large primary electron current. This large current, on the other hand, prevents resolution and can furthermore be harmful to sensitive electronic components. In addition, the amplifiers of the electronic signal processing circuits have only a relatively small bandwidth, so that potentials which change with a very high frequency cannot be imaged. (Scanning Electron Microsopy/1975 (Part i), IITRI Chicago, USA, April 1975, pages 456-471).
Also known is a device for pictorially displaying the potential distribution of high frequency signals at the surface of an electronic component stroboscopically, using the principle of a sampling oscilloscope which is equipped with a gating device for the primary electron beam. The pulsed primary electron beam is led across the component in lines by means of a deflection device. The pulses of the primary electron beams have a fixed phase relationship with respect to the potential at the measuring point and one obtains an image of the static potential distribution at the surface of the component at a predetermined phase. By shifting the phase, respective other images are obtained. While these individual images can be compared, it is not possible to see the instantaneous change of the potential pattern in time ("J. Sci. Instr." 1968, Series 2, Vol. 1, pages 595 to 600).